1. Field of the Invention
This invention relates to a method of forming a semiconductor film.
2. Description of the Related Art
Many phosphor-doped polysilicon films are used as a wiring material for semiconductor devices.
Published Unexamined Japanese Patent Application No. 2-34917 discloses a method of forming a low density phosphor-doped polysilicon film on a semiconductor wafer by the use of LPCVD. Further, Published Unexamined Japanese Patent Application No. 2-81432 discloses a technique of forming a film on a semiconductor wafer using LPCVD and organic materials.
The LPCVD is a method of supplying a reaction gas into a reaction tube while reducing pressure in the tube by exhausting air therefrom, thereby forming a film. This method is advantageous to form the above-polysilicon film, since an impurity concentration, a phosphor (P) concentration in this case, is controlled by adjusting the flow of a gas containing P to be doped.
FIG. 4 shows a relationship between the flow of dope gas and the growing rate of a film, under the condition that the pressure in the reaction tube of an LPCVD apparatus is fixed. The abscissa indicates the flow of a mixture of 99% He gas and 1% phosphine gas, while the ordinate indicates the growing rate of a film. FIG. 5 shows a relationship between the flow of dope gas and the density of phosphor contained in a thin film, under the same condition as above. The abscissa indicates the flow of the same mixture as above, while the ordinate indicates the density of phosphor. Each of the values in these graphs is a mean value of values obtained in given 25 points of the wafer surface. As is evident from the graphs, in the LPCVD method using Si.sub.2 H.sub.6, the density of the impurity can be controlled without changing the growing rate of a film, by adjusting the flow of the gas.
Apart from the above, to meet the recent demand for high integration and high reliability of a semiconductor device, the density of phosphor contained in a polysilicon film formed on the wafer must be uniform all over the film, the thickness of the film must be uniform, and films must be formed uniform between a plurality of wafers. In the LPCVD method, pressure in a reaction tube must be set at 0.05-0.3 Torr during growing films in order to enhance the uniformity of phosphor distribution.
Further, in a case where a phosphor-doped polysilicon film is formed using disilane serving as film forming gas and phosphine serving as dope gas, it is desirable to set the density of phosphine to about 1% in consideration of safety. Thus, if the required density of phosphor to be doped is high, a great amount of gas must be supplied. In this case, exhaustion efficiency must be greatly enhanced since the pressure in the reaction tube need be set at 0.05-0.3 Torr in a state where a great amount of gas is supplied therein.
However, the exhaustion efficiency has an upper limit for the following reasons:
In a vertical type CVD apparatus, for example, its housing for enclosing a reaction tube, and a mechanism for loading and unloading wafers, etc. is located between a clean room in which wafer cassettes are housed and a maintenance room in which a power control unit and a vacuum pump are installed. If the vacuum pump is arranged in the vicinity of the housing, it will be difficult to perform maintenance on the CVD apparatus, and further the vibration of the pump is transferred to wafers placed in the reaction tube, which raise dust. The dust may stick to the wafers and hence reduce the yield of products, so the pump must be located remote from the housing. Thus, an exhausting pipe extending between the vacuum pump and reaction tube inevitably has a low conductance, which requires a large capacity vacuum pump so as to provide high exhaustion efficiency. Moreover, a large diameter exhaustion pipe is required, resulting in an apparatus of a large size and a high cost.